The present invention relates to a spectrometer with a non-radioactive electron source for the chemical ionization of the substances under analysis. Such spectrometers include ion mobility spectrometers, electron capture detectors, and certain mass spectrometers. With this invention, possibly polluted gases can be analyzed and continuously monitored in a wide range of applications, for example in environmental analysis, in the control of chemical processes, and in civil and military applications to detect CWAs (chemical warfare agents) or explosives.
Ion mobility spectrometry is a method introduced in the 1970s for the highly sensitive detection of dangerous substances at low concentrations in air or other sample gases. An ion mobility spectrometer can be operated at atmospheric pressure and can be manufactured in a relatively compact form. Ion mobility spectrometers are therefore particularly suitable to be used as portable and mobile gas monitors and warning devices. Time-of-flight ion mobility spectrometers are the most widely used type. There are also “Aspiration Ion Mobility Spectrometers” from the Finnish company Environics Oy and “Asymmetric Field Ion Mobility Spectrometers” (FAIMS).
An ion mobility spectrometer generally consists of a reaction chamber, where ions of the substances under analysis (analyte ions) are generated, and a drift chamber, where the ions are separated according to their mobility in a drift gas. In FAIMS instruments they are separated according to the degree to which their mobility depends on the field strength. Radioactive electron emitting materials such as tritium, nickel-63, or americium-241 are usually used to ionize the substances. The disadvantage of radioactive ionization sources is that their use can be hazardous to the environment and dangerous to the health of the service personnel. Attempts have therefore been made to use non-radioactive electron sources such as photo-emitters or a corona discharge inside the reaction chamber. In both cases, however, experience has shown that the ionization processes which occur are not the same as with a radioactive ionization source, and so different species of analyte ions are produced or, in some cases, no analyte ions at all.
The patent specification by Budovich et al. (DE 196 27 621 C1) elucidates an ion mobility spectrometer that uses a non-radioactive electron source to produce electrons in an evacuated source chamber. An electric field accelerates the electrons, to 20 kiloelectronvolts for example, and they pass, from the source chamber through a window which is impermeable to gas and into a reaction chamber at atmospheric pressure, whereby they are partially decelerated. The electrons ionize the gas in the reaction chamber, as happens in the case of a radioactive electron source. The primary ions thus generated are the starting point for a chain of ionization reactions which ends with the substances under analysis also being ionized. The partition wall prevents the substances under analysis or the analyte ions from coming into contact with the electron source.
In the embodiments given in Budovich et al., the non-radioactive electron source is a photo-emitter or a thermal emitter, both of which require an operating pressure of less than 0.01 pascal in order to function. But ion mobility spectrometers do not generally have an integrated vacuum system which can be used to evacuate such a source chamber. In order for the non-radioactive electron source to have a commercially relevant operational lifetime, the gas permeability (leak rate) of the window should be low enough that the pressure increase in the source chamber is less than 10−10 pascal liters per second.
A window used by Budovich et al. consists of a mica disk approx. three to five micrometers thick, which withstands the pressure difference. The leak rate of the mica disk is sufficiently low. The ion currents in the ion mobility spectrometer, however, prove to be significantly smaller than those produced when a commercial radioactive electron source (nickel-63) with an activity of 100 megabecquerel, the currently permitted limit, is used. This is because the permeability of the mica disk to electrons with an energy of 20 kiloelectronvolts is low.
Electron sources are known from other fields of application which have silicon nitride windows that are permeable to electrons and impermeable to gas; these windows are less than 300 nanometers thick (Ulrich et al.: “Excitation of dense gases with low-energy electron beams”, in: Physikalische Blätter, 56 (2000), No. 6, Pages 49 to 52). If such windows are used in an ion mobility spectrometer at electron energies of about 20 kiloelectronvolts, more than a third of the electrons from the source chamber can reach the reaction chamber. At a diameter of about one millimeter, the thin windows withstand a pressure of one atmosphere. Apart from thickness and material, it is particularly the temperature of the window that is decisive for its gas permeability, which increases disproportionately to the window's temperature. The window heats up as the electrons pass through it because part of the electron energy always remains in the window. Therefore two opposing effects related to the thickness of the window must be taken into account in order to design a window with minimum gas permeability: on the one hand, thinner windows heat up less because they have better electron permeability; on the other hand, thicker windows are less permeable to gas than thinner windows.
Another patent specification of Budovich et al. (DE 196 27 620 C1) presents an electron capture detector with a non-radioactive electron source located in a source chamber and separated from the reaction chamber by a partition wall which is permeable to electrons but impermeable to gas. It is also possible to use such an ionization source in mass spectrometers.